The adenovirus host-cell reactivation assay, in which adenovirus 2 or 5 is treated with physical or chemical agents, and subsequently assayed for biological activity on monolayers of human fibroblasts, either capable of repairing ultraviolet damaged DNA or not, was used to study the interaction of several compounds on DNA. (Agents that make DNA damage repairable by ultraviolet repair mechanisms reduce the plaquing ability of adenovirus more rapidly if repair deficient (xeroderma pigmentosum) fibroblasts are used as viral hosts, than if normal ones are used.) Benzo(a)pyrene diol-epoxide I, the probable carcinogenic form of benzo(a)pyrene, when dissolved in tetrahydrofuran was found to inactivate adenovirus 5 30-fold more rapidly when the treated viruses were grown in xeroderma pigmentosum fibroblasts than when they were grown in normal fibroblasts. Moreover, one diol-epoxide I molecule bound to one virus appears to inactivate that virus when it effects xeroderma pigmentosum fibroblasts. A new ultraviolet light effect on the lambda phage - E. coli system was discovered, and a mutant of adenovirus 2 unable to grow in normal human fibroblasts, but able to grow in human tumor cells was discovered. BIBLIOGRAPHIC REFERENCE: Lytle, C.D., Day, R.S., III, Hellman, K.B., and Bockstahler, L.E.: Infection of UV-irradiated xeroderma pigmentosum fibroblasts by Herpes simplex virus: Study of capacity and Weigle reactivation. Mutat. Res. 36: 257-264, 1976.